Very low melt viscosity resin

ABSTRACT

A process is provided for the production of a very low melt viscosity (high melt flow index) polymer resin, suitable for use in spunbond or meltblown processing. According to the process of the current invention, a high melt viscosity (low melt flow index) resin is subjected to post-reactor molecular weight alteration by extrusion with a chemical prodegradant. The process produces a very low melt viscosity resin that can be used in spunbond or meltblown processing without further treatment to reduce the average molecular weight of the resin. Further, the very low melt viscosity resins produced according to the process of the current invention contain very little or no residual prodegradant.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/224,715, which was filed on Aug. 22, 2002 now U.S. Pat. No.6,855,777.

FIELD OF THE INVENTION

The present invention relates to polymer resins for use in spunbond andmeltblown processing. Specifically, the present invention relates to aprocess for the production of polymer resins with very low meltviscosity by post-reactor molecular weight alteration of high meltviscosity resins using prodegradants, where the final resins aresuitable for spunbond and meltblown processing without furtherpreparation and contain low concentrations of or no residualprodegradants.

BACKGROUND OF THE INVENTION

It is well known in the art that it is a desirable processing propertyfor polymer resins in spunbond and meltblown processing to have a lowviscosity when molten. For many commercial end-users, the melt-flowcharacteristics of standard, commercial polymer resins are not suitablebecause of their relatively high molecular weight, which results in ahigh melt viscosity. As low melt viscosity is desired, prior artreferences have sought to achieve low melt viscosity of the polymerthrough controlled scission of the polymer chain. This controlledscission, in effect, reduces the post-reactor average molecular weightof the polymer chains. As the average molecular weight is reduced, themelt viscosity is lowered. Furthermore, the molecular weightdistribution (MWD) is significantly altered.

It is well-known that polymer resins suitable for spunbond and meltblownprocessing may be produced by preparing polymer with a relatively highmelt viscosity and then subjecting it to a post-reactor molecular weightalteration using a chemical prodegradant, typically a free radicalinitiator, such as a peroxide. This degradation treatment occurs undersuch conditions that the melt viscosity of the polymer decreases to aspecific value. However, when producing a pelletized polymer for futureprocessing, this process has presented problems. U.S. Pat. Nos.4,451,589; 4,897,452 and 5,594,074 all report that when peroxidetreatment is used to produce a low melt viscosity polymer in a extrusionprocess, the resulting polymer is not easily pelletized. Specifically,the degraded polymer on exiting the extruder becomes so fluid and softthat it is difficult or impossible to cut into pellet form.

To avoid this problem, several processing techniques have used adegradation process involving a primary degradation wherein the averagemolecular weight of the polymer is reduced to a value above that desiredfor spunbond or meltblown processing. The degradation is performed in anextruder wherein an additional amount of prodegradant remainsimpregnated in the pelletized polymer for further degradation. Theadditional prodegradant acts to further reduce the average molecularweight to the desired value during meltblown or spunbond processing.U.S. Pat. No.5,594,074 to Hwo, et al, U.S. Pat. No. 4,451,589 to Morman,et al and U.S. Pat. No.4,897,452 to Berrier, et al all describeprocesses for making polymer pellets containing an unreacted freeradical initiator. Using the impregnated free radical initiator, thepolymers can be further degraded upon thermal treatment to form an ultralow melt viscosity polyolefin. U.S. Pat. No. 4,897,452 describes aprocess for the manufacture of propylene homopolymer or copolymerpellets in the presence of a primary and secondary free radicalinitiator, wherein the half-life of the second free radical initiator isat least twenty times longer than that of the first free radicalinitiator. In that invention at least 80% by weight of the second freeradical initiator, and not more than 20% by weight of first free radicalinitiator remain intact in the pellets and available for subsequentdecomposition during the conversion of the pellets into finishedarticles.

Another method consists of higher melt viscosity reactor granules thatare coated with peroxide so that they crack to lower melt viscosityduring spunbond processing. In all cases, the un-reacted peroxide cracksthe polymer to low melt viscosity during spunbond or meltblownprocessing.

However, materials having prodegradants either impregnated into orcoated onto pelletized polymer have some disadvantages. In particular,there is a danger that the residual prodegradant within the polymer willreact early, either before it gets to the end-user or before theend-user processes it. As a result, various lots of the polymer materialmay behave with a degree of inconsistency.

It is also known to produce polymer having a low melt viscosity directlyfrom an in-reactor process. In this case no post-reactor molecularweight alteration is required, as the desired melt viscosity property isproduced directly by the in-reactor polymerization of the monomer. Adraw back of resins produced by in-reactor processes is that they aresupplied in a flake rather than pellet form, resulting in the presenceof a significant amount of powdery fines, which create difficulties inhandling and transporting the material. Finally, in-reactor processingis not a viable option for a number of spunbond and meltblown fabricprocessors that lack the particular conveying systems necessary totransport materials supplied in flake form.

Therefore, it would be desirable to provide a process for producing apolymer resin that has low melt viscosity and good melt flow in spunbondand meltblown processing. A polymer resin produced by such a processwould have a low melt viscosity, as measured by melt flow index, incombination with a low residual content of prodegradant. Such materialwould be provided fully or nearly fully reacted prior to spunbond andmeltblown processing. Such material would also be provided in a pelletform for easy handling and transport.

SUMMARY OF THE INVENTION

The present invention provides a process for producing a high melt flowindex homopolymer or copolymer resin having a melt flow index of greaterthan 300 dg/min. and containing less than 300 ppm of residualprodegradant. The polymer resin produced according to the process of thepresent invention can be used to produce a fiber that can beincorporated into a non-woven fabric and can be processed on a spunbondor meltblown line to form a fabric (“web”) using standard commercialprocessing conditions and rates. The polymer resin of the currentinvention provides improved melt viscosity homogeneity during spunbondand meltblown processing relative to granules or pellets coated orimpregnated with prodegradants, such as free radical initiators.

The process of the current invention further provides improvedlot-to-lot consistency relative to products that contain substantialamounts of un-reacted peroxides or other prodegradants, which can reactduring shipping and storage, initiating degradation that results inunpredictable melt viscosity.

The process according to the current invention is applicable to avariety of polyolefin homopolymers and copolymers. In a preferredembodiment, the high melt flow index polymer resin produced is apolypropylene homopolymer, or random or block copolymer.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a process for producing polymer resins with lowmelt viscosity, suitable for spunbond or meltblown processing. Theprocess provides high melt flow index polymer resins having a melt flowindex of greater than 300 dg/min. Preferably, the melt flow index of theresin is from 300 dg/min. to 2500 dg/min. More preferably, polymersaccording to the current invention have a melt flow index of 300 to 1000dg/min. Polymers according to the current invention are generallysuitable for meltblown processing. Polymers according to the currentinvention having melt flow indices in the lower end of the stated range,e.g. less than 1000 dg/min., may also be suitable for spunbondprocessing. Additionally, the polymer resins produced according to theinvention contain less than 300 ppm of residual prodegradant,preferably, less than 50 ppm.

Further, polymer resins produced according to the invention haverelatively narrow molecular weight distributions (MWDs), as defined bythe function:MWD=Mw/Mnwhere: $\begin{matrix}{{Mw} = {{weight}\mspace{14mu}{average}\mspace{14mu}{molecular}\mspace{14mu}{weight}}} \\{{Mn} = {{simple}\mspace{14mu}{average}\mspace{14mu}{molecular}\mspace{14mu}{weight}}}\end{matrix}$

In general, the high melt flow index polymer resins produced accordingto the invention typically have molecular weight distributions of lessthan 3.0.

According to the process of the present invention a high melt flow indexpolymer resin is produced by extruding a low melt flow index polymerpowder with a prodegradant to initiate controlled degradation thatresults in a reduction of the average molecular weight of the polymer,providing a final product that has a melt flow index of greater than 300dg/min. and containing minimal residual prodegradant. According to oneembodiment of the invention, polymer reactor granules are combined withadditives. The polymer powder/additive blend is then fed into anextruder. The prodegradant is combined with the powder/additive duringextrusion by injecting it directly into the extruder, either at the feedthroat or through an opening in the barrel, preferably as a solution.According to an alternate embodiment, a prodegradant may be dry-blendedwith the polymer powder/additive blend before extrusion. Further, theadditives may be added as a solution with the prodegradant, by injectioninto the molten resin during extrusion. Regardless of how theprodegradant or additional additives are added, at the elevatedextrusion temperatures the prodegradant initiates controlled degradationthat decreases the average molecular weight of the polymer. Vacuumdevolitazation can be applied to the extruder barrel to remove anyunreacted prodegradant along with residual solvents. The resin leavesthe extruder through a die and is then quenched by a water bath andchopped into pellets. The molecular weight reduction obtained results ina very low melt viscosity, as measured by melt flow index.

According to an alternative embodiment of the invention, a low meltviscosity (high melt flow index) polymer resin may be produced through atwo stage process, which begins by performing a first stage extrusionprocess as described above, resulting in polymer pellets with a finalmelt flow index of approximately 300 to 700 dg/min. These pelletsthemselves are high melt flow index and may be suitable for spunbond ormelt blown processing, but may also be passed to a second stage processwhich is identical to the first stage except that the starting materialis the polymer pellet produced from the first stage processing.Specifically, the first stage polymer pellets of approximately 300 to700 dg/min melt flow index are fed into the extruder where they areextruded with a prodegradant and vacuum devolatized to remove residualprodegradant. The resin then proceeds to a water bath followed by dryingwith an air knife and then proceeds to a strand pelletizer. This secondstage extrusion process results in polymer pellets with a final meltflow index of 1000 dg/min. or greater and less than 300 ppm of residualprodegradant. As with the one stage process, the polymer may be drymixed with the prodegradant prior to extrusion.

Polymer resins that can be used as raw materials in the process of thecurrent invention typically have melt flow indices of 60 or greater, butthey may be as low as 0.7. Preferably, the prodegradant is added to theraw polymer resin in concentrations from 0.1 to 2.0 percent by weight,based on the weight of polymer. It will be apparent to those skilled inthe art that the process of the present invention is not limited to aparticular prodegradant or class of prodegradant. A number ofprodegradants, including free radical initiators, such as organicperoxides, are useful with the present invention. The class of organicperoxides includes, but is not limited to: TRIGONOX 101®(2,5-dimethyl-2,5-di-[tert-butylperoxyl]hexane) and TRIGONOX 301®(3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane), both availablefrom AKZO and (di-tert-amyl peroxide), available from CK Witco as DTAP®and from AKZO as Trigonox 201®. Additionally, a number of additives maybe used with the current invention, including, but not limited to:anti-oxidants, processing stabilizers, and acid scavengers. Examples ofadditives that are useful in the current invention are: IRGAFOS 168®(tris-[2,4-di-tert-butylphenyl]phosphite) and IRGANOX 1076®(octadecyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamate), both availablefrom CIBA, and zinc oxide and calcium stearate.

High melt flow index polymer resins produced according to the currentinvention contrast with commercial spunbond or meltblown resins, whichcontain an unreacted peroxide that initiates resin degradation duringspunbond or meltblown processing. The fully reacted resins produced bythe process of the current invention are expected to exhibit improvedmelt viscosity consistency over current commercial products.

EXAMPLES 1–5 One Step Process

Five samples of low melt viscosity polypropylene resin were producedusing the single extrusion process. The initial melt flow indices (MFIs)of the resins put into the process were from 0.7 to 60. Table 1 showsthe properties of the resins that were input into these five trials.

TABLE 1 Molecular Weight Distributions of Starting Materials starting MnMw material MFI (Kg/mole) (Kg/mole) Mw/Mn granules 0.7 82 473 5.9granules 18 44 205 4.7 granules 60 37 155 4.15

Examples were run using 30 mm, 43 mm and 240 mm extruders. The quantityof peroxide fed to the extruder varied from 0.31 to 1.2 weight percent.The polypropylene powder was dry-blended with a peroxide and fed to thehopper of the extruder. For the trials on the 43 mm extruder, the barreltemperature at the hopper was set to 350° F. and increased along thebarrel to 450° F. at the vacuum port, which was located just upstream ofthe die. The die temperature was set to 375° F. After extrusion, thesamples were quenched and pelletized. Table 2 details the properties ofthe low melt viscosity polymers produced in each trial.

TABLE 2 Processing Conditions and Properties Example 1 2 3 4 5 Extruder30 mm 30 mm 240 mm 43 mm 43 mm starting MFI 0.7 60 18 18 18 final MFI1600 1500 1000 1400 2210 residual peroxide <50 ppm <50 ppm <50 ppm 15ppm 25 ppm Mn 30 24 34 24 22.5 Mw 57 58 91 57 51

Extruder barrel temperature settings are critical to forming a productthat contains minimal un-reacted prodegradant. The prodegradantdecomposition rate (i.e. the rate at which the prodegradant initiatescontrolled degradation of the polymer) is specified by its half-life,which decreases exponentially as temperature increases. The processtemperature must be high enough to provide a half-life that issubstantially shorter than the residence time of the extruder. Ingeneral, the residence time of the material in the extruder should be atleast five times the half-life of the prodegradant. The residence timeis determined by the extruder size, screw design, and throughput. Thethroughput rate and devolatilization vacuum pressure were varied tomeasure the effects of those parameters on product molecular weight andresidual prodegradant. The data in Table 3 indicate that for the aboveexamples the best residual peroxide levels were obtained using the 43 mmextruder.

TABLE 3 Extrusion Conditions for Producing Resins with the Desired MeltFlow Viscosity screw speed throughput vacuum residence half-life (rpm)(kg/hr) (in Hg) time (s) Die T (F.) (sec) 150 27 5 41 390 6

EXAMPLES 6 AND 7

Two samples of high melt viscosity resin were produced frompolypropylene pellets produced by extruding polypropylene homopolymerreactor granules in the presence of a peroxide to induce controlledreduction of the average molecular weight. One of the products had amelt flow of 300 dg/min and the other had a melt flow of 600 dg/min. Themolecular weight distributions of the low melt viscosity pelletsproduced are provided in

TABLE 4 Molecular Weight Distributions of Spunbond Materials starting MnMw material MFI (Kg/mole) (Kg/mole) Mw/Mn pellets 300 36 111 3.1 pellets600 34 96 2.9

These low melt viscosity materials were further processed in a secondstep to produce materials suitable for meltblown processing. Theprocessing conditions and properties for the low melt viscositypolypropylene resins produced in these trials is shown in Table 5.

TABLE 5 Processing Conditions and Properties Extruder 43 mm 43 mmstarting MFI 340 (pellet) 643 (pellet) final MFI 1503 1470 residualperoxide 160 ppm 75 ppm Mn 24.8 23.7 Mw 60 59 Mw/Mn 2.4 2.5

The foregoing examples using polypropylene homopolymers have beenprovided for illustrative purposes only and should not be construed aslimiting the scope of the invention. Those skilled in the art willrecognize that the process of the current invention can be applied to avariety of block and random copolymers of polypropylene and otherpolymers. The process according to the current invention has beenpracticed successfully with polymers of both standard and highisotacticity. Additionally, the prodegradants and additive packages usedin the examples are only for illustrative purposes. The process of thecurrent invention can be used successfully with various prodegradantsand additive packages. The full scope of the invention will be clear tothose skilled in the art from the claims appended hereto.

1. A process for producing a high melt flow index polymer, said highmelt flow index polymer having a melt flow index of at least 300 dg/minand containing less than 300 ppm of a residual prodegradant, saidprocess comprising the steps of: providing a low melt flow indexpolymer; adding at least one additive to said low melt flow indexpolymer; adding at least one prodegradant to said low melt flow indexpolymer; extruding said low melt flow index polymer in an extruder at anelevated temperature to initiate controlled molecular weight reductionof said low melt index polymer to form a high melt flow index polymer;and quenching and pelletizing said high melt flow index polymer.
 2. Theprocess of claim 1, wherein said at least one prodegradant is dry mixedwith said low melt flow index polymer prior to said extruding.
 3. Theprocess of claim 1, wherein said at least one prodegradant is added tosaid low melt flow index polymer as a solution during said extruding. 4.The process of claim 3, wherein said at least one prodegradant is addedby injecting at the feed throat of an extruder.
 5. The process of claim3, wherein said at least one prodegradant is added by injecting into thebarrel of an extruder.
 6. The process of claim 1, wherein said high meltflow index polymer contains less than 50 ppm of a residual prodegradant.7. The process of claim 1, wherein said at least one prodegradantcomprises an organic peroxide.
 8. The process of claim 1, wherein saidat least one additive is selected from the group consisting of:anti-oxidants, processing stabilizers, and acid scavengers.
 9. Theprocess of claim 1, further comprising the step of removing unreactedprodegradant from said high melt flow index polymer.
 10. The process ofclaim 9, wherein said unreacted prodegradant is removed by vacuumdevolitazation.
 11. The process of claim 10, wherein said vacuumdevolitazation is accomplished by applying a vacuum to the barrel ofsaid extruder during extruding.
 12. The process of claim 1, wherein saidlow melt flow index polymer has a melt flow index of 100 dg/min or less.13. The process according to claim 1, wherein the residence time of thepolymer in said extruder is less than 60 seconds.
 14. The process ofclaim 1, wherein said extruding is performed at a temperature from about325° F. to about 475° F.
 15. The process of claim 1, wherein saidprodegradant is added in a quantity of from about 0.2 to about 2.0percent by weight, based on the weight of low melt index polymer. 16.The process of claim 1, wherein said low melt flow index polymer is apolypropylene.
 17. The process of claim 16, wherein said polypropyleneis a random or block copolymer.
 18. The process of claim 16, whereinsaid polypropylene has a high isotacticity.
 19. The process of claim 1,wherein said high melt flow index polymer has a molecular weightdistribution of from about 1.9 to about 2.9.
 20. A pelletized high meltflow index polymer having a melt flow index of at least 300 dg/min andcontaining a residual amount of prodegradant that is less than 300 ppm;said high melt flow index polymer being produced by controlleddegradation of a low melt flow index polymer; said controlleddegradation being accomplished by extruding a low melt flow indexpolymer with at least one prodegradant.
 21. The pelletized high meltflow index polymer according to claim 20, having a melt flow index offrom 300 dg/min to 1000 dg/min.
 22. The pelletized high melt flow indexpolymer according to claim 20, containing less than 50 ppm of residualprodegradant.
 23. The pelletized high melt flow index polymer accordingto claim 20, having a molecular weight distribution of less than 3.0.24. The pelletized high melt flow index polymer according to claim 20,wherein said polymer is a polypropylene.
 25. The pelletized high meltflow index polymer according to claim 24, wherein said polypropylene isan ethylene/propylene copolymer.
 26. The pelletized high melt flow indexpolymer according to claim 24, wherein said polypropylene has a highisotacticity.